tests: Fix a -Wbad-function-cast warning in the atomic tests

Assigning the gpointer return value from g_atomic_pointer_get() to a
gsize variable was not making -Wbad-function-cast happy. Assign to an
intermediate gpointer variable and then cast that instead.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1475
diff --git a/glib/tests/atomic.c b/glib/tests/atomic.c
index 2aafe86..84c13fb 100644
--- a/glib/tests/atomic.c
+++ b/glib/tests/atomic.c
@@ -92,7 +92,8 @@
   g_assert (ip == 0);
 
   g_atomic_pointer_set (&gs, 0);
-  gs2 = (gsize) g_atomic_pointer_get (&gs);
+  vp2 = g_atomic_pointer_get (&gs);
+  gs2 = (gsize) vp2;
   g_assert (gs2 == 0);
   res = g_atomic_pointer_compare_and_exchange (&gs, 0, 0);
   g_assert (res);